Experimental and computational approaches to investigate the high oxidation state oxo- and hydroxido- redox chemistry of osmium

Van Niekerk, Daniel Malan Emmanuel (2016-03)

Thesis (PhD)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: The OsVIII oxo/hydroxido complexes that are abundant in mild to relatively concentrated basic aqueous solutions are OsVIIIO4, [OsVIIIO4(OH)]- and two cis-[OsVIIIO4(OH)2]2- species. OsVIII complexes that contain water ligands are thermodynamically unfavoured w.r.t. the abovementioned complexes. OsVIIIO4 reacts with hydroxide in two, consecutive, elementary coordination sphere expansion steps to form the [OsVIIIO4(OH)]- complex and then the cis-[OsVIIIO4(OH)2]2- species. The thermodynamic driving force of the first reaction is the bonding energy of the OsVIII-OH metal-hydroxido ligand, while of the second reaction it is the larger hydration energy of the doubly-charged cis-[OsVIIIO4(OH)2]2- in comparison with that of the two, singly-charged reactants. The DFT-calculated ΔG°rxn in the simulated aqueous phase (COSMO) is -2.42 kcal.mol-1 for the first reaction and -0.61 kcal.mol-1 for the second reaction (PBE-D3 functional) and agree to within 1 kcal.mol-1 with reported values, at -2.69 and 0.33 kcal.mol-1, respectively. The thermodynamically most stable paramagnetic OsVII product species in a 2.0 M NaOH aqueous matrix are the five-coordinate trans-[OsVIIO3(OH)2]- and octahedral mer-[OsVIIO3(OH)3]2- stereoisomers. The thermodynamic driving force of the coordination sphere expansion reaction of trans-[OsVIIO3(OH)2]- with a hydroxide ion to form mer-[OsVIIO3(OH)3]2- is the larger hydration energy of the doubly-charged mer-[OsVIIO3(OH)3]2- complex in comparison with that of the two, singly-charged reactants. The thermodynamically most stable OsVI species obtained via DFT is the diamagnetic trans-[OsVIO2(OH)4]2- species and the spin state of this species was confirmed with Evans method, 1H NMR spectroscopy experiments. DFT-calculated metal-ligand bond lengths of the global energy minimum geometry of trans-[OsVIO2(OH)4]2- correlates well with reported X-ray crystal structure data. QTAIM and EDA analyses indicate that the OsVIII=O, OsVI=O and OsVII=O bonding interactions are ionic (closed-shell) and that OsVIII-OH, OsVI-OH and OsVII-OH bonding interactions are polar covalent (dative) while NCI analysis indicates that relatively weak intramolecular hydrogen bonding interactions occur between neighbouring oxo and hydroxido ligands in the [OsVIIIO4(OH)]- and cis-[OsVIIIO4(OH)2]2- species. Diamagnetic OsVIII oxo/hydroxido, [OsVIIIO4(OH)n]n- (n = 1, 2), (of d0 electron configuration) and trans-[OsVIO2(OH)4]2- species (d2) react in a 1:1 mole ratio, in a 2.0 M NaOH aqueous matrix, to form paramagnetic OsVII product species (d1), equation 1. [Os ] + [Os ] 2 [Os ] 1 The forward (comproportionation) reaction, equation 1, is first order w.r.t. the OsVIII and OsVI concentrations while the reverse (disproportionation) reaction is second order w.r.t. the OsVII concentration. From non-linear least squares fits of stopped-flow UV-Vis spectroscopy kinetic traces, rate constants, thermodynamic activation energies and standard reaction energies were determined. The forward reaction is endothermic whilst the activation enthalpy of the reverse reaction is negative. A kinetic isotope effect (KIE) for the forward and reverse reactions, equation 1, are both approximately 1.1 providing evidence that the ratedetermining step of the reactions coincide with the transfer of a proton. DFT-calculated standard reaction energies for the OsVI & OsVIII comproportionation reaction at 298.15 K (e.g. PBE, ΔH°rxn, ΔS°rxn and ΔG°rxn are 21.13 kcal.mol-1, 71.06 cal.mol-1.K-1 and -0.06 kcal.mol-1, respectively) compare well with experimentally determined observed values (ΔH°rxn(obs), ΔS°rxn(obs) and ΔG°rxn(obs) are 17.1  1.2 kcal.mol-1, 61.0  4.3 cal.mol-1.K-1 and -1.1  2.5 kcal.mol-1, respectively). The DFT-calculated, most energetically favoured comproportionation reaction mechanism consists of (i) the formation of a (singlet state) adduct, [OsVIII=O···HO-OsVI]3-, (ii) ‘spin-forbidden’, concerted electron-proton transfer (EPT) from the OsVIII monomer to the trans-[OsVIO2(OH)4]2- monomer to form a (triplet state) dimer, [OsVII-OH···O=OsVII]3-, (iii) separation of the OsVII monomers and ultimately (iv) interconversion of the separated species to form a combination of the trans-[OsVIIO3(OH)2]- and mer-[OsVIIO3(OH)3]2- stereoisomers. EPT from the OsVIII monomer to the OsVI monomer is the rate-determining step of the reversible comproportionation reaction, corroborating the experimental evidence (KIE). Good agreement between experimental and DFT-calculated activation energies were obtained for the stepwise EPT reaction pathway. Sequential electron proton transfer (ET-PT or PT-ET) mechanisms for equation 1 are shown, via DFT calculations, not to occur in a 2.0 M NaOH matrix at 298.15 K. From non-linear least squares fits of UV-Vis spectroscopy kinetic traces, rate constants, thermodynamic activation energies and standard reaction energies for the reduction of OsVIII with methanol (CH3OH or CD3OH) in a 2.0 M NaOH aqueous matrix were determined. Cleavage of an α-C-H bond in CH3OH and the cleavage of an α-C-D bond in CD3OH are the rate-determining step for the two reactions. The relatively large KIE value, kH/kD, of approximately 11.82 (298.15 K) obtained is indicative of a normal, primary isotope effect. The complete, five-step DFT-calculated reaction pathway consists of (i) non-covalent dimer formation of the reactants, (ii) hydrogen atom transfer (HAT) of an α-C-H bond of CH3OH to cis-[OsVIIIO4(OH)2]2-, (iii) PT of the O-H hydrogen of ˙CH2OH to mer-[OsVIIO3(OH)3]2-, (iv) ET from ˙CH2O- to trans-[OsVIIO4(OH)2]-, and (v) separation of the products. The organic and inorganic monomers involved in the elucidated reaction mechanism do not separate from one another between steps (ii) to (iv), but stay bound to one another via intermolecular hydrogen bonding interactions in order to firstly, decrease the PT/ET transfer distance between them, and secondly, to avoid the formation of highly energetically unfavourable cationic, anionic or radical monomer species. The mechanistic study agreed with our experimentally determined result that the rate-determining step of the reaction involves the cleavage of an α-C-H bond in CH3OH. Fair agreement between activation energies of DFT-calculated (PBE-D3 functional), 19.1 kcal.mol-1 (Δ‡H°), -38.1 cal.mol-1.K-1 (Δ‡S°) and 30.5 kcal.mol-1 (Δ‡G°), and experimentally determined, 14.4  1.2 kcal.mol-1 (Δ‡H°), -12.5  4.1 cal.mol-1.K-1 (Δ‡S°) and 18.1  2.4 kcal.mol-1 (Δ‡G°), parameters were obtained.

AFRIKAANSE OPSOMMING: Die OsVIII oxo/hydroxido komplekse wat volop is in matig tot relatief gekonsentreerde basiese waterige oplossings is OsVIIIO4, [OsVIIIO4(OH)]- en twee cis-[OsVIIIO4(OH)2]2- spesies. OsVIII komplekse wat water ligande bevat is termodinamies ongunstig t.o.v. die bogenoemde komplekse. OsVIIIO4 reageer met hidroksied in twee agtereenvolgende, elementêre stappe naamlik; koördinasie sfeer uitbreiding na die [OsVIIIO4(OH)]- kompleks gevolg deur die cis-[OsVIIIO4(OH)2]2- spesie. Die termodinamiese dryfkrag van die eerste reaksie is die bindingsenergie van die OsVIII-OH metaal-hydroxido ligand, terwyl van die tweede reaksie is dit die groter hidrasie energie van die dubbel-gelaaide cis-[OsVIIIO4(OH)2]2- in vergelyking met dié van die twee, een-een-gelaaide reaktante. Die DFT-benaderde ΔG°rxn in die gesimuleerde waterige fase (COSMO) is -2.42 kcal.mol-1 vir die eerste reaksie en -0.61 kcal.mol-1 vir die tweede reaksie (PBE-D3 funksies) en stem ooreeen binne 1 kcal.mol-1 met rapporteerde waardes, teen -2.69 en 0.33 kcal.mol-1 onderskeidelik. Die termodinamies mees stabiele paramagnetiese OsVII produk spesies in 'n 2.0 M NaOH waterige matriks is die vyf-koördineerde trans-[OsVIIO3(OH)2]- en oktaëdriese mer-[OsVIIO3(OH)3]2- stereoisomere. Die termodinamiese dryfkrag van die koördinasie sfeer uitbreiding reaksie van trans-[OsVIIO3(OH)2]- met 'n hidroksiedioon om mer-[OsVIIO3(OH)3]2- te vorm is die groter hidrasie energie van die dubbel-gelaaide mer-[OsVIIO3(OH)3]2- kompleks in vergelyking met dié van die twee, een-een-gelaaide reaktante. Die termodinamies mees stabiele OsVI spesies verkry via DFT is die diamagnetiese trans-[OsVIO2(OH)4]2- spesies. Die spin toestand van hierdie spesie is bevestig deur middel van die Evans metode, 1H-KMR spektroskopie eksperimente. DFT-berekene metaal-ligand bindings lengtes van die globale energie minimum geometrie van trans-[OsVIO2(OH)4]2- korreleer goed met gerapporteerde X-straal kristalstruktuur data. QTAIM en EDA ontledings dui daarop dat die OsVIII=O, OsVI=O en OsVII=O binding interaksies ioniese is (‘closed-shell’) en dat OsVIII-OH, OsVI-OH en OsVII-OH binding interaksies polêr kovalente (datief) is, terwyl NCI analise dui aan dat relatief swak intramolekulêre waterstofbinding interaksie plaas vind tussen naburige oxo en hydroxido ligande in die [OsVIIIO4(OH)]- en cis-[OsVIIIO4(OH)2]2- spesies. Diamagnetiese OsVIII oxo/hydroxido, [OsVIIIO4(OH)n]n- (n = 1, 2), (van d0 elektronkonfigurasie) en trans-[OsVIO2(OH)4]2- spesies (d2) reageer in 'n 1:1 mol verhouding, in 'n 2.0 M NaOH waterige matriks, om paramagnetiese OsVII produk spesies (d1) te vorm, vergelyking 1. [Os ] + [Os ] 2 [Os ] 1 Die vorentoe (komproportionasie) reaksie, vergelyking 1, is eerste orde met betrekking tot die OsVIII en OsVI konsentrasies terwyl die omgekeerde (disproporsionerings) reaksie tweede orde is met betrekking tot die OsVII konsentrasie. Van nie-lineêre kleinste kwadratiese passings op stop-vloei, UV-Vis spektroskopiese kinetiese spore is; reaksie tempos, termodinamiese aktiveringsenergieë en standaard reaksie energië bepaal. Die voorwaartse reaksie is endotermies terwyl die aktivering entalpie van die terugwaartse reaksie negatief is. Die kinetiese isotoop effek (KIE) vir die voorwaartse en terugwaartse reaksies, vergelyking 1, is albei ongeveer 1.1, ter bewyse dat die tempo-beperkende stap van die reaksies ooreenstem met die oordrag van 'n proton. DFT-berekende standaard reaksie energië vir die OsVI & OsVIII komproportionasie reaksie by 298.15 K (bv PBE, ΔH°rxn, ΔS°rxn en ΔG°rxn is 21.13 kcal.mol-1, 71.06 cal.mol-1.K-1 en -0.06 kcal.mol-1 onderskeidelik) en vergelyk goed met eksperimenteel waargeneemde waardes (ΔH°rxn(obs), ΔS°rxn(obs) en ΔG°rxn(obs) is 17.1  1.2 kcal.mol-1, 61.0  4.3 cal.mol-1.K-1 en -1.1  2.5 kcal.mol-1 onderskeidelik). Die DFT-berekende, mees energie voordelige meganisme vir komproportionasie bestaan uit (i) die vorming van 'n (‘singlet-state’) nie-kovalente dimeer, [OsVIII=O···HO-OsVI]3-, (ii) 'n spin-verbode elektronproton oordrag (EPT) uit die OsVIII monomeer met die trans-[OsVIO2(OH)4]2- monomeer om 'n tweede (‘triplet-state’) nie-kovalente dimeer te vorm, [OsVII-OH···O=OsVII]3-, (iii) die skeiding van die OsVII monomere en uiteindelik (iv) van die onderling geskeide spesies om 'n kombinasie van die trans-[OsVIIO3(OH)2]- en mer-[OsVIIO3(OH)3]2- stereoisomere te vorm. EPT van die OsVIII monomeer na die OsVI monomeer is die tempo-beperkende stap van die omkeerbare komproportionasie reaksie, ter ondersteuning van eksperiment (KIE). Goeie ooreenkoms tussen eksperimentele en DFT-berekende aktiveringsenergieë is verkry vir die stapsgewyse EPT reaksie pad. Die sekwensiële elektron proton oordrag (ET-PT of PT-ET) meganismes vir vergelyking 1 vind nie, via DFT berekeninge, plaas in 'n 2.0 M NaOH matriks teen 298.15 K nie. Vanuit nie-lineêre kleinste kwadratiese passings op UV-Vis spektroskopiese kinetiese spore, is reaksie tempos, termodinamiese aktiveringsenergieë en standaard reaksie energie vir die reduksie van OsVIII met metanol (CH3OH of CD3OH) in 'n 2.0 M NaOH waterige matriks bepaal. Klowing (Cleaving) van 'n α-C-H binding in CH3OH en die klowing van 'n α-C-D binding in CD3OH is die tempo-beperkende stap vir die twee reaksies. Die relatiewe groot KIE waarde kH/kD, van ongeveer 11.82 (298.15 K) verkry is 'n aanduiding van 'n normale, primêre isotoop effek. Die volledige vyf-stap-DFT-berekende reaksie pad bestaan uit (i) nie-kovalente dimeer vorming van die reagense, (ii) waterstofatoom oordrag (HAT) van 'n α-C-H binding van CH3OH na cis-[OsVIIIO4(OH)2]2-, (iii) PT van die O-H waterstof van ˙CH2OH na mer-[OsVIIO3(OH)3]2-, (iv) ET van ˙CH2O- na trans-[OsVIIO4(OH)2]- en (v) die skeiding van die produkte. Die organiese en anorganiese monomere betrokke in die meganisme bly gebind aan mekaar tussen stappe (ii) tot (iv), via intermolekulêre waterstofbinding interaksies om eerstens, PT/ET oordrag afstand te verminder, en tweedens, om die vorming van hoogs energie ongunstige kationiese, anioniese of radikale monomeer spesies te voorkom. Die meganistiese studie stem ooreen met ons eksperimentele bepaling, dat die tempo-beperkende stap van die reaksie behels die klowing van 'n α-C-H binding in CH3OH. Billike ooreenkoms tussen aktivering energie van DFT-bereken (PBE-D3 funksionele), 19.1 kcal.mol-1 (Δ‡H°), -38.1 cal.mol-1.K-1 (Δ‡S°) en 30.5 kcal.mol-1 (Δ‡G°), en eksperimenteel bepaal, 14.4  1.2 kcal.mol-1 (Δ‡H°), -12.5  4.1 cal.mol-1.K-1 (Δ‡H°) en 18.1  2.4 kcal.mol-1 (Δ‡H°), parameters is verkry.

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